Adaptive circuits for purposes of this invention am those whose transfer function can be varied. The process of varying the transfer function can be performed in several ways. In one arrangement, the circuit initially operates with one or more parameters and the parameter values can be individually varied in a manner intended to improve circuit operation. In another arrangement, the circuit operates with an initial set of parameters and this entire set is changed for another parameter set upon some predetermined condition.
Digital receivers use adaptive circuits, such as equalizers, and those which provide timing recovery and/or carrier recover, in the regeneration of data from a received signal which has propagated through a communications channel. Within the receiver, the adaptive circuit provides a transfer function which compensates for the characteristics of the communications channel and thereby reduces errors in the data recovery process. Typically, each adaptive circuit varies its transfer function in response to a first error signal derived from the received signal.
The adaptation or adjustment of the transfer function of a circuit used in the data recovery process can be performed in two different arrangements. In "blind" adaptation, the circuit's transfer function is varied during reception of a signal conveying customer data whose value at any time is not known. In contrast, a training sequence including one or more a prod known data values can be transmitted in certain time intervals and the transfer function of the adaptive circuit is varied so as to minimize a first error signal. This first error signal at any time is equal to the difference between the recovered data value at that time and the corresponding a priori known transmitted data value.
There are several problems associated with the use of training sequences. First, the transmission of the training sequences requires an interruption in the transmission of customer data. Second, the use of training sequences alone does not provide adaptation during customer data transmission. Therefore, the adaptation is not continuous and the transfer function provided by the adaptive circuit does not track variations in the characterstics of the communications channel during customer data transmission. These characteristics may render adaptation solely in response to training sequences impractical in applications where the communications channel characteristics vary frequently. Third, if the adaptation process is to be performed at times other than during predesignated time intervals, such as system start-up, a communications channel must be provided from the receiver back to the transmitter in order to inform the latter to commence training sequence transmission.
The use of blind adaptation also has associated problems. A significant problem is that referred to as "false convergence". This situation arises in adaptive circuits in certain communications systems when the adaptive algorithm adjusts a circuit's transfer function to a "settled" state "believing" that circuit performance has been optimized when, in fact, it has not. In these situations, the system is referred to as having multiple solutions, one or more of which significantly improve circuit performance and one or more of which do not. This problem is troublesome and it would be desirable if a technique could be devised which overcomes the problem of false convergence in the recovery of data from a received signal.